ADC Differential mode of the ATMEGA644

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#1
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Hello everyone,

I'm gonna explain my problem in several steps:

What I want to do: measure a voltage difference between two pins of a sensor (max différence 5V)

Problem: I can't fix no pin from the sensor to the GND or VCC, the sensor being already powered by another device which GND is not the same as mine. Moreover, I don't want to disturb the output of this sensor (as you can guess...)

My question: what would be the problem if I use the differential mode of the ATMega644 to know the voltage difference between the two sensor pins? What happens if one of the two pins is higher than my VCC / lower than my GND?

More info: this sensor is actually a variable resistor being powered by another device (voltage divider). This other device (a car ECU) doesn't have the same ground as me.

Thanks a lot for you help :)

My electronic projects blog >> www.limpkin.fr

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Quote:

What happens if one of the two pins is higher than my VCC / lower than my GND?

Not good. You violate Absolute Maximum Ratings, and can cause current flow through the protection diode(s).

The real sparkies will give the correct answer, but I think you need some signal conditioning/level shifting.

You can put lipstick on a pig, but it is still a pig.

I've never met a pig I didn't like, as long as you have some salt and pepper.

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The car puts out 0-5v ground referenced. Just connect avr gnd to car gnd, read 0-5v direct? Too easy?

Imagecraft compiler user

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Yep - too easy :) Have a look at the problem statement again - the car is running two isolated electrical systems without a common ground (and whatever this unit does, it mustn't connect them together)

@limpkin - is this race car instrumentation related?

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I guess you need TWO avr's each one measuring the voltage of ONE of the sensor pins to ground, each avr having it's OWN ground to measure the sensor. Then use an opto isolator to couple the two avr's together so one can report its' reading to the other. The master unit will compute the result from the two readings.

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@Martin: yes :)
@ksharf: yes indeed

actually i just found the pinout of a system that does more or less what i want to do... and it seems that the ground of the sensors is the ground of the car!
so apparently i don't have any problem....

My electronic projects blog >> www.limpkin.fr